This invention relates to the drying of granular solids. More particularly, it relates to a method and apparatus for removing water from granular solids in a more inexpensive and efficient manner. The phrase "granular solid," as used herein, refers to any regularly or irregularly shaped solid or semisolid particles, fibers, wafers, flakes, crystals or the like which can adsorb or absorb water and which can be dried by the application of heat by passage of a hot gas stream through a volume of the solid.
Granular solids often have an unacceptably high water content. One standard drying technique passes a stream of hot gas through the solids to desorb water followed by a cool stream to lower the solids' temperature. In this conventional method, the cooling gas is introduced when the effluent stream from the drying solid shows a decrease in water vapor concentration or an increase in temperature.
U.S. Pat. No. 4,324,564 discloses the "Four Front" method, an improved technique of operating sorption beds. As disclosed therein, regeneration of sorption beds occurs through the medium of moving fronts or regions in the sorption bed where changes in sorbent loading, temperature, and sorbate content of the gas occur. During regeneration, introduction of a hot gas stream into the bed creates a desorption front. This front, designated therein as an RW front, is bounded on its downstream side by bed conditions characteristic of equilibrium between the sorbent material and fluid waste ("W", sorbate-rich effluent), and on its upstream side by bed conditions characteristic of equilibrium between the hot sorbent material and hot regenerant gas ("R"). Upon subsequent introduction of cooling fluid, another front is created, designated therein as a thermal front or a "PR" front, which moves more rapidly than the RW front. The thermal front can arise in several ways. When the hot regenerant gas contains a substantial concentration of sorbate (i.e., water) and the coolant is substantially sorbatefree (assumed for purposes of discussion to have characteristics similar to dry product gas "P"), a PR transition is created comprising a faster stripping front which effects removal of all or most of the sorbate on the hot bed in equilibrium with the hot regenerant fluid, and a thermal front which effects the major temperature transition. Upstream of this transition, the bed is in equilibrium with coolant (herein designated "P"), while downstream, the bed is in equilibrium with hot regenerant. If sorbate is present in the coolant, multiple fronts are produced which together form the PR transition, including one front which constitutes the major thermal front. When the regeneration and cooling are conducted using substantially sorbate-free gas, the PR transition is a pure thermal wave bounded on its downstream side by bed conditions characteristic of equilibrium between the sorbent and the hot regenerant gas, and on its upstream side by bed conditions characteristic of equilibrium between the sorbent and the cooling gas.
U.S. Pat. No. 4,324,563 disclosed that a number of operational advantages including energy saving can be achieved by timing the introduction of the cooling gas prior to the breakthrough of the midpoint of the RW front. The preferred operation of the Four Front method for sorption bed regeneration introduces the cooling gas so that the thermal component of the PR transition is in the last third of the bed or most preferably at the bed exit when the slower RW front is at or breaking through the bed exit.
Prior art solids drying apparatus use conventional techniques rather than the Four Front method so more heat is needed to dry the solids than when the Four Front method is used.
Objects of the invention include a method of drying granular solids which is more energy and cost efficient than the methods previously used. Another object is to decrease heat consumption by using a portion of the sensible heat contained in the granular solids to assist in drying. A further object is to provide apparatus for drying granular solids in a more energy efficient manner. A still further object of the invention is to develop a system for continuous energy efficient solids drying.
Another object is to provide a sorption bed in apparatus for drying granular solids which removes moisture from the cooling gas stream and enables use of a stripping front to remove all or most of the water in the solids that was in equilibrium with the hot regenerant gas.
These and other objects and features of the invention will be apparent from the following description and the drawing.